High-Strength Collagen Sponge

ABSTRACT

To provide a collagen sponge excellent in mechanical strength and a production method for the collagen sponge. A collagen sponge including a porous construct having a pore structure, the collagen sponge having a tensile strength of 1 N or more and 5 N or less in every direction including a length direction and a width direction. The collagen sponge may be produced by a production method including the following steps: (1) a step of subjecting a collagen solution obtained by mixing collagen and a solvent to stirring and deaeration treatment; (2) a step of subjecting the collagen solution to freeze-dry treatment; and (3) a step of subjecting a dried collagen product after the freeze-dry treatment to insoluble treatment.

TECHNICAL FIELD

The present invention relates to a collagen sponge including a porousconstruct having a pore structure, and more specifically, to a collagensponge having strong tensile strength in every direction. In addition,the present invention also relates to a production method for thecollagen sponge. Further, the present invention also relates to atherapeutic method for a cartilage tissue using the collagen sponge.

BACKGROUND ART

A meniscus of a knee joint is a fibrocartilage tissue lying between afemur in the joint and a tibia of a lower leg, and is a tissue to befrequently subjected to a large mechanical load. The meniscus plays arole in shock absorption, load distribution, improvement in ability toslide, stabilization of the joint, or the like. The meniscus is ofteninjured in sports or damaged by daily life activities, and the damagecauses a pain of the knee joint and motion limitations. The meniscus issupplied with blood only at an outer peripheral portion thereof. Whenthe portion at which blood is supplied is torn, the torn portion ishealed naturally in some cases but generally needs to be treated. Inaddition, when a portion at which blood is not supplied is torn, thetorn portion is not healed naturally and needs to be treated.

Hitherto, damaged meniscus has been treated by conservative therapy,such as drug therapy or exercise therapy, or treated by surgery, inaccordance with torn sites or damage states. As the surgery treatment,excision or partial excision of the meniscus has been performed.Although, such surgery can reduce pain temporarily, functions of themeniscus remain impaired, resulting in arthrosis deformans or the like.In recent years, with an improvement in endoscopic technology,functional preservation has been performed by suture of the meniscusunder arthroscopic view. However, such suture has a problem in that thesuture is not applicable to damage with a defect, complex damage,degenerative tear, and the like, and hence functions of the meniscuscannot be repaired. Such problem is desired to be solved in treatment ofnot only the meniscus but also all cartilage tissues that are hardlyhealed naturally owing to a small number of blood streams.

As a solution to the problem described above, regenerative medicine hasbeen investigated actively. For example, in Patent Literature 1, thereis a disclosure of a “method involving culturing cartilage cells on asubstrate (collagen sponge) for cell culture and implanting the culturedcells into a defect together with the substrate. ” However, in PatentLiterature 1, the method involves inoculating the cells into thesubstrate, culturing the cells in vitro for a certain period, and thenimplanting the substrate into a tissue defect, and hence has a problemin that the method can be performed only in a facility with advancedequipment, such as a cell processing center. The method also has aproblem of having a difficulty in ensuring safety.

In Patent Literature 2, there is a disclosure of a “collagen substrateto be used as a graft.” However, in Patent Literature 2, the substratedoes not have a pore structure in its surface and inside, which inhibitscell infiltration into the inside. The substrate has a small surfacearea to be brought into contact with a surrounding tissue and a smallbinding force to a tissue, and hence its bonding to an implantation siteis difficult and long-term fixation with a suture thread or the like isrequired in order to prevent dropping thereof.

A patient to whom a substrate derived from collagen has been implantedinto a tissue to be subjected to a mechanical load is required to stayin bed until the substrate is bonded to the implantation site.Therefore, a substrate to be implanted into a tissue to be subjected toa mechanical load is required to have a structure easily bonded to theimplantation site and to have physical properties equivalent to those ofa tissue into which the substrate is to be implanted so that a patientsubjected to implantation can resume a daily life immediately. Further,the substrate is considered to be deteriorated and modified with time invivo, and hence in order to prevent second surgery for the patient, thesubstrate is required to be decomposed in a certain period afterimplantation into a living body and replaced by a normal autologoustissue.

Collagen Meniscus Implant (ReGene Biologics Inc.) is sold as a meniscussubstitute in the United States of America. The meniscus substitute usesnative collagen as a raw material, and hence has a problem of lowbiocompatibility. ACTIFIT (trademark) (Orteq Ltd) is sold in Europe, butis formed of biodegradable polyurethane, and hence has a problem in thatcells infiltrate thereinto but hardly engraft. ACTIFIT (trademark) isdecomposed in about half a year to about a year after transplantation,but in a decomposed portion, the meniscus is not regenerated and adefective state remains. There is a strong need for a substitute thathas high biocompatibility, and when substituted in a meniscus defectiveportion, allows surrounding cells to infiltrate thereinto, is decomposedover time, and allows reconstruction of a meniscus tissue along with thedecomposition.

In Patent Literature 3, there is a disclosure of a “collagen sponge thathas a stress of from 10 kPa to 30 kPa when loaded with 10% strain, hasin its surface and inside a pore structure having an average porediameter ranging from 50 μm to 400 μm, and has a pore diameter standarddeviation equal to or less than 40% of the average pore diameter.” InPatent Literature 3, there is a disclosure of a method involvingadjusting a high-concentration atelocollagen solution to a neutral pH toprecipitate atelocollagen fibrils, and molding the atelocollagensolution in this state into a desired shape, to thereby produce thecollagen sponge. The collagen sponge of Patent Literature 3 has highcompressive strength, but has weak tensile strength owing to generationof a variation in density of the dispersed state of the collagenfibrils. In addition, a collagen sponge having directionality inorientation of fibrils has high physical strength in one direction, buthas weaker strength in any other direction.

In addition, a sponge-like product has been produced in the followingmanner: a collagen solution is concentrated, air-dried, and neutralizedand warmed to produce a collagen construct having formed thereincollagen fibrils, and the collagen construct is freeze-dried (PatentLiterature 4). Collagen fibril formation occurs only when a pH, atemperature, and a salt concentration are adjusted to physiologicalconditions. The collagen sponge of Patent Literature 4 has a fibrousstructure in which collagen is oriented unidirectionally.

In a tissue to be subjected to a mechanical load, such as the meniscus,a defective portion is deformed by being subjected to a force, and hencethere has been a need for suture in substitution with a substituteformed of collagen. However, the substitute formed of collagen has thefollowing problem: the substitute is markedly reduced in tensilestrength when infiltrated by water in a body fluid, physiologicalsaline, or the like, and hence suture cannot be performed because, evenwhen the defective portion is sutured, the substitute collapses from aportion threaded with a suture thread. In surgery of the meniscus underarthroscopic view, which has been becoming mainstream in recent years,the arthroscopic view is filled with physiological saline, and when thesubstitute is inserted into a suture portion, the substitute is passedthrough an inside of a trocar (intubation) filled with physiologicalsaline, by threading the substitute with a suture thread and pulling thesuture thread. The related-art collagen sponge has not been practicalbecause of a problem in that the related-art collagen sponge is markedlyreduced in tensile strength when impregnated with water in blood, a bodyfluid, physiological saline, or the like, and hence cannot be passedthrough the inside of the trocar by pulling the sewing suture thread.

CITATION LIST Patent Literature

[PTL 1] JP 2008-79548 A

[PTL 2] JP 08-38592 A

[PTL 3] JP 5909610 B2

[PTL 4] U.S. Pat. No. 5,256,418 A

SUMMARY OF INVENTION Technical Problem

An object of the present invention is to provide a collagen spongeexcellent in mechanical strength and a production method for thecollagen sponge.

Solution to Problem

The inventors of the present invention have made investigations in orderto achieve the above-mentioned object, and as a result, have found thata collagen sponge can be produced in a state in which collagen moleculesare uniformly dispersed, by adjusting a collagen solution to an acidicpH and subjecting the collagen solution to stirring and deaeration, andthe produced collagen sponge has a uniform pore structure and hasapproximately constant physical strength when a force is applied fromany direction. Thus, the inventors have completed the present invention.

That is, the present invention includes the following.

-   1. A collagen sponge, including a porous construct having a pore    structure, the collagen sponge having a tensile strength of 1 N or    more and 5 N or less in any direction.-   2. The collagen sponge according to the above-mentioned item 1,    wherein the collagen sponge has been subjected to insoluble    treatment with a chemical cross-linking agent.-   3. The collagen sponge according to the above-mentioned item 1 or 2,    wherein the collagen sponge has an average pore diameter falling    within a range of 1 μm or more and less than 50 μm.-   4. The collagen sponge according to any one of the above-mentioned    items 1 to 3, wherein the collagen sponge has a stress of 7 kPa or    more and 30 kPa or less when loaded with 10% strain.-   5. The collagen sponge according to any one of the above-mentioned    items 1 to 4, wherein the collagen sponge includes a freeze-dried    product of an acidic collagen solution.-   6. A collagen sponge, which is obtained by applying insoluble    treatment with a chemical cross-linking agent to a porous construct    having a pore structure obtained by freeze-drying a collagen    solution subjected to stirring and deaeration treatment, the    collagen sponge having a tensile strength of 1 N or more and 5 N or    less and having a stress of 7 kPa or more and 30 kPa or less when    loaded with 10% strain.-   7. The collagen sponge according to the above-mentioned item 6,    wherein the collagen solution includes a collagen solution obtained    without undergoing a fibrillogenesis step.-   8. A production method for a collagen sponge, including the    following steps:

(1) a step of subjecting a collagen solution obtained by mixing collagenand a solvent to stirring and deaeration treatment;

(2) a step of subjecting the collagen solution to freeze-dry treatment;and

(3) a step of subjecting a dried collagen product after the freeze-drytreatment to insoluble treatment.

-   9. The production method for a collagen sponge according to the    above-mentioned item 8, wherein the collagen solution in the    step (2) includes a collagen solution obtained without undergoing a    fibrillogenesis step.-   10. The production method for a collagen sponge according to the    above-mentioned item 8 or 9, wherein the collagen solution has an    acidic pH.-   11. The production method for a collagen sponge according to any one    of the above-mentioned items 8 to 10, wherein the step (1) includes    the following step (1-1) and step (1-2):

(1-1) a step of preparing a collagen solution by adding collagen to asolvent, followed by stirring and deaeration treatment; and

(1-2) a step of subjecting the collagen solution injected into acontainer to stirring and deaeration treatment, to thereby reduce airbubbles in the collagen solution.

-   12. The production method for a collagen sponge according to any one    of the above-mentioned items 8 toll, wherein the stirring and    deaeration treatment is performed at a revolutional centrifugal    force of 1 G or more and 600 G or less and a rotational centrifugal    force of 0.1 G or more and 80 G or less.-   13. A substitute for cartilage treatment, including the collagen    sponge of any one of the above-mentioned items 1 to 7 as a    substrate.-   14. The substitute for cartilage treatment according to the    above-mentioned item 13, wherein the cartilage treatment includes    fibrous cartilage regeneration treatment.-   15. The substitute for cartilage treatment according to the    above-mentioned item 13, wherein the cartilage treatment includes    meniscus regeneration treatment.-   16. A therapeutic method for cartilage, including implanting the    collagen sponge of any one of the above-mentioned items 1 to 7 into    cartilage.-   17. The therapeutic method for cartilage according to the    above-mentioned item 16, wherein the cartilage includes fibrous    cartilage.-   18. The therapeutic method for cartilage according to the    above-mentioned item 17, wherein the fibrous cartilage includes a    meniscus.-   19. The collagen sponge according to any one of the above-mentioned    items 5 to 7, wherein the collagen solution includes an    atelocollagen solution.-   20. The production method for a collagen sponge according to any one    of the above-mentioned items 8 to 12, wherein the collagen solution    includes an atelocollagen solution.-   21. The collagen sponge according to any one of the above-mentioned    items 1 to 7, wherein the collagen sponge has a thickness of 1 mm or    more and 10 mm or less.

Advantageous Effects of Invention

The collagen sponge of the present invention has approximately constanttensile strength in any direction, and has excellent tensile strengtheven under moist condition. The collagen sponge of the present inventioncan maintain its tensile strength even when exposed to a body fluid orwater, and has such physical strength that, even when a defectiveportion is sutured, the collagen sponge is not broken from a portionthreaded with a suture thread to fall off, and endures the suture of atissue, thereby being able to beheld in the tissue. In addition, thecollagen sponge of the present invention is also excellent incompressive strength, and hence the pore structure therein does notcollapse and allows surrounding cells to infiltrate thereinto, andbesides, the collagen sponge does not apply a load (physical stimulus)to a surrounding tissue when transplanted, and hence is suitable for useas a substrate for implantation into a cartilage tissue defectiveportion. According to the production method of the present invention, acollagen sponge having excellent physical strength can be easilyproduced. In addition, the production method of the present inventioncan deal with a large amount of an acidic solution of collagen havinghigh viscosity and high concentration, and hence can efficiently producea collagen sponge having a desired size.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an image taken of a cross-section of a collagen sponge of thepresent invention with an electron microscope VHX-D510 (KeyenceCorporation) at a magnification of 100×. Black portions of the imagecorrespond to pores of the collagen sponge (Example 2).

FIG. 2 is an image taken of a cross-section of a related-art collagensponge with an electron microscope VHX-D510 (Keyence Corporation) at amagnification of 100×. Black portions of the image correspond to poresof the collagen sponge (Example 2).

FIG. 3 are explanatory views of a method of measuring the tensilestrength of a collagen sponge (Example 2).

FIG. 4 is a graph for showing the results of measurement of the tensilestrength of the collagen sponge of the present invention and the tensilestrength of the related-art collagen sponge (Example 2).

FIG. 5(A1) is an image taken of the collagen sponge of the presentinvention with a scanning electron microscope (SEM) (Hitachi SU6600, 7kV) at a magnification of 100×. FIG. 5(A2) is an image taken of acollagen sponge obtained through a fibrillogenesis step with a scanningelectron microscope (SEM) (Hitachi SU6600, 10 kV) at a magnification of100×. FIG. 5(B1) is an image taken of the collagen sponge of the presentinvention with a scanning electron microscope (SEM) (Hitachi SU6600, 7kV) at a magnification of 10, 000×. FIG. 5(B2) is an image taken of thecollagen sponge obtained through a fibrillogenesis step with a scanningelectronmicroscope (SEM) (Hitachi SU6600, 10 kV) at a magnification of10,000×. FIG. 5(C1) is an image taken of the collagen sponge of thepresent invention with a transmission electronmicroscope (TEM)(JEOL1400Plus) at amagnification of20,000×. FIG. 5(C2) is an image takenof the collagen sponge obtained through a fibrillogenesis step with atransmission electron microscope (TEM) (JEOL 1400Plus) at amagnification of 10,000× (Reference Example 1).

DESCRIPTION OF EMBODIMENTS

(Collagen Sponge)

A collagen sponge is a porous construct using collagen as a material andhaving a pore structure having a plurality of pores. The collagen spongeof the present invention has a tensile strength of 1 N or more and 5 Nor less, which is required in the procedure of cartilage treatment. Thetensile strength of the collagen sponge of the present invention ispreferably 1.5 N or more and 5 N or less. The collagen sponge of thepresent invention has strong and excellent tensile strength in anydirection. The collagen sponge of the present invention is produced bydirectly subjecting a collagen solution in a state in which collagenfibrils have not been formed to freeze-dry treatment, and hence thesponge is considered to be formed with collagen molecules in a randomarrangement. By virtue of the collagen molecules being oriented randomlyinstead of being oriented in one direction, the tensile strength isexcellent in any direction. When the tensile strength is less than 1 N,strength required in the suture of a tissue in cartilage treatment isnot satisfied. In addition, when the tensile strength is more than 5 N,stress required in cartilage treatment (stress of 7 kPa or more and 30kPa or less at the time of loading with 10% strain) is affected. Inaddition, when the tensile strength is 1 N or more and 5 N or less inany direction, the collagen sponge is not broken from a portion having ahole for a suture thread and is held in a tissue even when pulled invarious directions by suture.

In addition, herein, the tensile strength is preferably tensile strengthunder moist condition. When the tensile strength under moist conditionis 1 N or more and 5 N or less, sufficient strength and handleabilitycan be obtained in a solution or in vivo. The tensile strength of thecollagen sponge under moist condition may be evaluated by immersing thecollagen sponge in physiological saline at 37° C., and then measuringits tensile strength while keeping the moist state. A test for thetensile strength may be performed by any method, and may be performed inaccordance with a conventional method.

Herein, the tensile strength refers to a force (N) at break in the casewhere a cylindrical test piece having a diameter of 30 mm and athickness (height) of 5 mm brought into moist condition is pulled in aradial direction. When the collagen sponge is pulled, the collagensponge may be pulled by making holes therein and threading each of theholes with a thread. More specifically, a force (N) at break (at thetime of start of breakage) in the following case is measured: two siteseach 5 mm from the center on a straight line passing through the centerof a smooth surface of the collagen sponge are each threaded with asuture thread, and a test piece in the state of being threaded with thesuture threads is infiltrated with physiological saline, followed bypulling of the suture threads on both sides threading the test piece ata rate of 10 mm/min (see Example 2 and FIG. 3). When measured asdescribed above, the tensile strength of the collagen sponge of thepresent invention is 1 N or more and 5 N or less, preferably 1.5 N ormore and 5 N or less in any direction.

The collagen sponge of the present invention may be a collagen spongethat have been subjected to insoluble treatment with a chemicalcross-linking agent. The insoluble treatment increases the physicalstrength of the collagen sponge, and extends a remaining period in atissue into which the collagen sponge has been transplanted. Theinsoluble treatment cross-links collagen molecules in a randomorientation, and increases the mechanical strength of the collagensponge in all directions.

The insoluble treatment with a chemical cross-linking agent is performedby bringing the whole of a freeze-dried product of a collagen solution(hereinafter sometimes referred to as dried collagen product) intocontact with the chemical cross-linking agent without deforming thedried collagen product. Examples of the chemical cross-linking agentinclude a water-soluble chemical cross-linking agent and a vaporablechemical cross-linking agent. The insoluble treatment with awater-soluble chemical cross-linking agent may be performed by immersingthe dried collagen product in the water-soluble cross-linking agent. Theinsoluble treatment with a vaporable chemical cross-linking agent may beperformed by placing the dried collagen product and the chemicalcross-linking agent (e.g., a formalin solution) in a sealed container.The chemical cross-linking agent is preferably the water-solublechemical cross-linking agent. Preferred water-soluble chemicalcross-linking agents are as described later.

The collagen sponge of the present invention has a uniform porestructure. The average pore diameter of the collagen sponge of thepresent invention falls within the range of 1 μm or more and less than50 μm, preferably the range of 5 μm or more and 30 μm or less. Inaddition, the pore diameter standard deviation of the collagen sponge ofthe present invention is 20 μm or less, preferably 15 μm or less, morepreferably 7 μm or less. In the collagen sponge of the presentinvention, a value obtained by dividing a value of the pore diameterstandard deviation by the average pore diameter (value of pore diameterstandard deviation/average pore diameter) is 0.7 or less, preferably 0.6or less. The average pore diameter and the standard deviation may becalculated by randomly selecting a plurality of (e.g., 100) pores fromthe surface of the collagen sponge, measuring the long diameter of eachof the pores, and defining the long diameter as the diameter of thepore. The collagen sponge of the present invention has a small averagepore diameter and a dense pore structure as compared to a related-artcollagen sponge. The collagen sponge of the present invention has adense and uniform pore structure, and hence can obtain a high tensilestrength in any direction. When the average diameter is 1 μm or less,cells cannot infiltrate, and hence properties required in cartilagetreatment are not obtained. Also when the average diameter is 50 μm ormore and the standard deviation is 20 μm or more, or the value obtainedby dividing the value of the pore diameter standard deviation by theaverage pore diameter is more than 0.7, the tensile strength and thestress become non-uniform, and hence properties required in cartilagetreatment are not obtained.

The collagen sponge of the present invention has a stress of 7 kPa ormore and 30 kPa or less, preferably 10 kPa or more and 25 kPa or less,more preferably 15 kPa or more and 20 kPa or less when loaded with 10%strain. The stress has the same meaning as the compressive strength. Thestress at the time of loading with 10% strain is measured by immersingthe collagen sponge of the present invention in physiological saline at37° C., and using a small desktop testing machine EZ-S (ShimadzuCorporation). The collagen sponge of the present invention has bothexcellent tensile strength and compressive strength (stress) equivalentto that of the related-art collagen sponge. The stress required incartilage treatment is 7 kPa or more and 30 kPa or less, and when thestress is 7 kPa or more, the pore structure for allowing cells toinfiltrate can be prevented from being crushed by compression by thesurrounding tissue of an implantation site. In addition, when the stressof the collagen sponge is 30 kPa or less, the stress of the collagensponge can be prevented from being higher than that of the tissue intowhich the collagen sponge is to be implanted, and hence a physicalstimulus to the surrounding tissue can be suppressed and prevented fromcausing inflammation or the like.

The collagen sponge of the present invention may be a freeze-driedproduct of an acidic collagen solution. The acidic collagen solution isan acidic solution in which collagen is dissolved in a solvent. Apreferred pH is a pH of 1 or more and 4.5 or less, more preferably a pHof 2 or more and 4 or less, still more preferably a pH of 2.5 or moreand 3.5 or less. The acidic collagen solution may contain an additive.The additive is suitably, for example, one that does not promotefibrillogenesis of the collagen. The acidic collagen solution is uniformthroughout the entirety of the solution, has a uniform dispersed stateof collagen molecules, and does not have collagen fibrils formedtherein. The freeze-dried product of the acidic collagen solution is aproduct obtained by freeze-drying the acidic collagen solution, has auniformpore structure, has more uniform compressive strength and tensilestrength, and has properties of being strong against tension from anydirection and being strong against compression from any direction. Thefreeze-dried product does not have collagen fibrils formed therein, andhas collagen molecules oriented randomly instead of being oriented inone direction.

A collagen sponge according to one embodiment of the present inventionis a collagen sponge obtained by applying insoluble treatment with achemical cross-linking agent to a porous construct having a porestructure obtained by freeze-dry a collagen solution subjected tostirring and deaeration treatment, the collagen sponge having a tensilestrength of 1 N or more and 5 N or less and having a stress of 7 kPa ormore and 30 kPa or less when loaded with 10% strain. The collagensolution subjected to stirring and deaeration treatment is a uniformcollagen solution obtained by mixing collagen and a solvent throughstirring and deaeration, in which air bubbles have been reduced. Detailsof the stirring and deaeration treatment are described later.

The collagen solution is preferably a collagen solution obtained withoutundergoing a fibrillogenesis step. The collagen undergoesfibrillogenesis when a pH, a temperature, and a salt concentration areadjusted to physiological conditions. Herein, the fibrillogenesis stepis a step of equilibrating the collagen solution to the physiologicalconditions, specifically, for example, 0.9% NaCl, phosphate bufferedsaline pH 7.4 (PBS), and 0.02 M Na₂HPO₄.

(Production Method for Collagen Sponge)

The collagen sponge of the present invention is produced by a productionmethod including at least the following steps (1) to (3):

-   (1) a step of subjecting a collagen solution obtained by mixing    collagen and a solvent to stirring and deaeration treatment;-   (2) a step of subjecting the collagen solution to freeze-dry    treatment; and-   (3) a step of subjecting a dried collagen product after the    freeze-dry treatment to insoluble treatment.

As the collagen to be used as a material for the collagen sponge of thepresent invention, there may be used: insoluble collagen collected froma tissue in vivo, such as tendon collagen derived from the Achillestendon or collagen derived from the skin; or soluble collagen orsolubilized collagen, such as enzyme-solubilized collagen(atelocollagen), alkali-solubilized collagen, acid-soluble collagen, orsalt-soluble collagen, and in particular, the atelocollagen ispreferred. The species of animal from which the collagen is derived isnot particularly limited, and any collagen having such a denaturationtemperature that the collagen does not denature by heat during culturemay be used without problems. Specifically, there may be used collagenderived from a mammal, such as cow or pig, collagen derived from a bird,such as chicken, or collagen derived from fish, such as tuna or tilapia.Recombinant collagen may also be used. In the collagen to be used as amaterial for the collagen sponge of the present invention, a side chainof a constituent amino acid of the collagen may be subjected to chemicalmodification. A specific example thereof is collagen subjected toacylation, such as acetylation, succinylation, or phthalation,alkylation, such as methylation or ethylation, or esterification.

The step (1) includes subjecting a collagen solution obtained by mixingcollagen and a solvent to stirring and deaeration treatment. The“collagen solution obtained by mixing collagen and a solvent” only needsto be a solution in which the collagen and the solvent coexist with eachother, and may be in a state immediately after mixing of the collagenand the solvent in which the collagen has not been dissolved in thesolvent, or may be in a state in which the collagen has been dissolvedin the solvent. The step (1) includes the following step (1-1) and/orstep (1-2):

-   (1-1) a step of preparing a uniform high-concentration collagen    solution by adding collagen to a solvent, followed by stirring and    deaeration treatment;-   (1-2) a step of subjecting the collagen solution injected into a    container to stirring and deaeration treatment, to thereby reduce    air bubbles in the collagen solution.

The step (1) preferably includes both the step (1-1) and the step (1-2).

In addition, in the present invention, in order to preventfibrillogenesis of collagen molecules in the collagen solution to besubjected to the step (2), it is required that the collagen solution benot brought to the physiological conditions during the procedure of thestep (1).

In the step (1-1), the mixture obtained by mixing the collagen and thesolvent is subjected to stirring and deaeration treatment todissolve/mix the collagen in the solvent, and thus a uniformhigh-concentration collagen solution can be prepared. The solvent forpreparing the collagen solution may be any solvent having an acidic pH,and for example, hydrochloric acid, acetic acid, citric acid, or malicacid may be used. In addition, the collagen solution may contain anadditive to the extent that the purpose of the present invention is notimpaired. The additive is suitably, for example, one that does notpromote fibrillogenesis of the collagen. The collagen solution preparedby subjecting the mixture to the stirring and deaeration treatment hasan acidic pH. More specifically, the pH of the collagen solution is a pHof 1 or more and 4.5 or less, preferably a pH of 2 or more and 4 orless, more preferably a pH of 2.5 or more and 3.5 or less. Also in thestep (1-2), the collagen solution has an acidic pH. More specifically,the pH of the collagen solution is a pH of 1 or more and 4.5 or less,preferably a pH of 2 or more and 4 or less, more preferably a pH of 2.5or more and 3.5 or less. When the pH of the collagen solution at thetime of molding is acidic, collagen molecules dissolve in the solution,the entire solution has a uniform concentration, and the dispersed stateof the collagen molecules becomes a uniform state. Thus, a state inwhich collagen fibrils are not formed can be established. With this, thepore structure of the collagen sponge can be made more uniform, and acollagen sponge whose compressive strength and tensile strength are bothmore uniform can be produced. Hitherto, a high-concentration collagensolution has been adjusted to a neutral pH to precipitate collagen andused in this state for the production of a collagen sponge, but therehas been a disadvantage in that the dispersed state of the collagenbecomes non-uniform through the generation of a variation in density,resulting in weak tensile strength of the produced collagen sponge.

In the present invention, the collagen solution is freeze-dried toproduce a dried collagen product through the step (2) to be describedlater. The collagen concentration of the collagen solution serving as araw material for the freeze-dry is preferably 50 mg/ml or more and 110mg/ml or less, 60 mg/ml or more and 100 mg/ml or less, 70 mg/ml or moreand 90 mg/ml or less, or 75 mg/ml or more and 85 mg/ml or less. When thecollagen concentration of the collagen solution is 50 mg/ml or more and110 mg/ml or less, a collagen sponge having physical properties similarto those of a cartilage tissue can be obtained. Particularly whenatelocollagen is used, the collagen concentration is desirably 70 mg/mlor more and 90 mg/ml or less. The use of the solution having a collagenconcentration of 50 mg/ml or more and 110 mg/ml or less reducesdifferences in physical properties from the cartilage in vivo, and whenthe collagen sponge is transplanted after inoculation of cartilage cellsthereinto, it is also possible to culture the inoculated cells whilebearing a load to be applied to cartilage cells and a cartilage tissuein vivo. The physical strength of the collagen of the present inventionmay also be obtained by insoluble treatment.

In the step (1-2), the collagen solution is uniformized to reduce airbubbles in the collagen solution. The air bubbles in the collagensolution may be reduced by stirring and deaeration treatment. Thecollagen solution may be subjected to the stirring and deaerationtreatment in a container or mold having a desired shape. The containerhaving a desired shape is a molding container for a collagen sponge, andhas a shape suited for the production of, for example, a cylindrical orcubic collagen sponge. The collagen sponge may be, after its production,used by being further cut into a desired shape at the time of actualuse, or the collagen sponge may be produced using the container or moldhaving a desired shape in the first place. A method of using thecontainer or mold having a desired shape in the first place is notparticularly limited, but when the collagen sponge is to be transplantedinto a cartilage defective portion, it is desired that the mold beproduced so as to suit the shape of the cartilage defective portion. Asa specific method, a mold having a shape corresponding to the shape of adefective portion may be made by optical modeling based on CT or MRIdata of a patient himself.

In the present invention, the stirring and deaeration treatment is usedas means for preparing a uniform high-concentration collagen solution inthe step (1-1), and as means for uniformizing the collagen solution toreduce air bubbles in the solution in the step (1-2). The stirring anddeaeration may be performed by using a revolution/rotation-type stirringand deaeration apparatus based on a planetary motion. The principle ofthe revolution/rotation-type stirring and deaeration apparatus is asfollows: a container containing an object to be treated is rotated whilebeing revolved, and thus, through the utilization of a centrifugal forceaction, the object to be treated is moved outward and stirred, and atthe same time, deaerated by pushing out an included gas in the oppositedirection. Further, because the container rotates while revolving, aspiral flow (eddy) is generated in the object to be treated in thecontainer to increase the stirring action. A high-concentration collagensolution has high viscosity, and hence is difficult to prepare in auniform state, but can be made uniform by performing the stirring anddeaeration.

In the revolution/rotation-type stirring and deaeration apparatus, thecontainer revolves around a predetermined revolution axis while rotatingaround a central axis (rotation axis). In the present invention, anangle between the plane of revolution of the container and the rotationaxis is from 40 degrees to 50 degrees, preferably 45 degrees. Theabove-mentioned angle allows the stirring and deaeration to be performedsatisfactorily. The angle may be appropriately set depending on theshape of the container and other conditions.

In the revolution/rotation-type stirring and deaeration apparatus, arevolution speed, a rotation speed, and a ratio therebetween are notparticularly limited. The revolutional centrifugal force is 1 G or moreand 600 G or less, preferably 400 G or more and 600 G or less. Therotational centrifugal force is 0.1 G or more and 80 G or less.

In addition, the revolutional centrifugal force, the rotationalcentrifugal force, and a ratio therebetween may be changed depending onthe viscosity of the collagen solution. When the revolutionalcentrifugal force falls within the above-mentioned range, a sufficientcentrifugal force can be applied to the collagen solution in thecontainer, and hence bubbles can be efficiently removed from thecollagen solution. In addition, the case in which the rotationalcentrifugal force falls within the above-mentioned range is preferredbecause the rotation of the container exhibits a high stirring effect onthe collagen solution, and collagen molecules are dispersed withoutbecoming non-uniform. When a high-concentration collagen solution issubjected to general centrifugal deaeration instead ofrevolution/rotation-type stirring and deaeration, a centrifugal forcecapable of deaeration causes the collagen molecules to be non-uniformthrough a centrifugal action, resulting in unevenness in collagenconcentration and structure after freeze-dry.

The operation of the apparatus may be performed under the atmosphericpressure, but is preferably performed under reduced pressure in order tothoroughly perform the deaeration within a short period of time. Thecontainer is generally a tubular container, and specifically, asterilized tubular container made of stainless steel, polyethylene,Teflon (trademark), or the like is used. A temperature during thestirring and deaeration is 4° C. or more and 40° C. or less, preferably4° C. or more and less than 30° C. When the temperature during thestirring and deaeration is set to 40° C. or less, denaturation of thecollagen can be prevented. A stirring and deaeration time greatly variesdepending on the properties, such as viscosity, of the collagen solutionand the shape and size of the container, but may be set to from about 1minute to about 30 minutes in the step (1-1), and may be set to from 60seconds to 120 seconds, preferably 90 seconds in the step (1-2). Inorder to suppress the generation of a concentration gradient of thecollagen molecules, it is preferred that the stirring and deaeration beperformed within a short period of time. For the stirring anddeaeration, about 10 seconds to about 40 seconds of stirring anddeaeration may be performed a plurality of times, and for example, inthe case of the step (1-1), 30 seconds of stirring and deaerationtreatment may be performed 10 times for a total of 5 minutes.

(Freeze-Dry Step)

The collagen solution after the stirring and deaeration is immediatelyfreeze-dried (step (2)). The collagen solution before the freeze-dry ispreferably a solution obtained without undergoing a fibrillogenesisstep. Through the freeze-dry, a dried collagen product having a spongystructure is produced. A method for the freeze-dry involves firstfreezing the collagen solution in a molding container for a collagensponge, and then drying the resultant under reduced pressure, and afreeze-dry technique known per se may be used. Examples of the methodfor the freeze-dry include rapid freezing and slow freezing. The poresize of the dried product varies depending on the freezing method, andhence a freezing method enabling a desired pore size is selected. Forexample, the pore size is reduced by the rapid freeze-dry, and the poresize is increased by the slow freeze-dry. The pore size is desirablysuch that cells can infiltrate into the inside of the collagen spongeand adhesion of the cells is not inhibited by a body fluid flowing intoand out of the collagen sponge. Therefore, the pore size is desirablyadjusted so as to achieve an average pore diameter of 1 μm or more andless than 50 μm, preferably 5 μm or more and 30 μm or less. In thedrying under reduced pressure, the boiling point of the frozen productis lowered by the reduced pressure, and the inside of the frozen productis dried by subliming the water in the frozen product at a lowtemperature. In the present invention, the drying under reduced pressuremay be performed by setting a starting temperature to from −40° C. to−15° C., and increasing the temperature to normal temperature over time.Specifically, the desired pore size may be obtained by leaving acontainer filled with the collagen solution to stand still in afreeze-dry machine cooled to −30° C. to freeze the solution, and thendrying the resultant under reduced pressure for from 70 hours to 75hours while increasing the temperature over time from −30° C. to normaltemperature.

(Insoluble Treatment Step)

In the present invention, the step (3), i.e., insoluble treatment of thedried collagen product can increase the physical strength of the driedcollagen product, and adjust the remaining period of the collagen spongein a tissue into which the collagen sponge has been transplanted. Whenthe insoluble treatment is performed, the insoluble treatment needs tobe performed uniformly into an inside of the dried collagen productwithout deforming the dried collagen product. The insoluble treatment inthe present invention is desirably treatment with a chemicalcross-linking agent, dry-heat treatment, or γ-ray irradiation treatment,which enables insoluble treatment into the inside of the dried product.Examples of the chemical cross-linking agent include a water-solublechemical cross-linking agent and a vaporable chemical cross-linkingagent. It is considered that the insoluble treatment can cross-linkcollagen in a random orientation, and can increase the mechanicalstrength of the collagen sponge.

Reaction conditions for each insoluble treatment are adjusted so thatthe finished product has a tensile strength of 1 N or more and 5 N orless and has a stress of 7 kPa or more and 30 kPa or less when loadedwith 10% strain. The dry-heat treatment may be performed by achieving acompletely dried state and then leaving the dried product to stand for30 minutes or more under a heating atmosphere of about 120° C. The γ-rayirradiation treatment may be performed by giving moisture to the driedproduct to the extent that the product is not swollen and thenperforming irradiation at 0.1 kGy or more. The insoluble treatment witha water-soluble chemical cross-linking agent may be accomplished byimmersing the dried collagen product in an aqueous solution containingthe water-soluble chemical cross-linking agent. For example, whenglutaraldehyde is used, the dried collagen product is immersed in anaqueous solution containing glutaraldehyde at a concentration of 0.5%.In addition, in the insoluble treatment with a vaporable chemicalcross-linking agent, the insoluble treatment maybe performed in a sealedcontainer by placing the dried collagen product and the chemicalcross-linking agent, such as a formalin solution, in the sealedcontainer. The water-soluble chemical cross-linking agent is, forexample, an aldehyde compound or an epoxy compound, more preferably anepoxy compound. Specifically, there may be used, for example, ethyleneglycol diglycidyl ether, allyl glycidyl ether, 2-ethylhexyl glycidylether, phenyl glycidyl ether, phenol (EO) 5 glycidyl ether,p-tert-butylphenyl glycidyl ether, dibromophenyl glycidyl ether, laurylalcohol (ED) 15 glycidyl ether, resorcinol diglycidyl ether, neopentylglycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, glycerinpolyglycidyl ether, trimethylolpropane polyglycidyl ether,pentaerythritol polyglycidyl ether, diglycerin polyglycidyl ether,polyglycerin polyglycidyl ether, sorbitol polyglycidyl ether, diglycidylterephthalate, diethylene glycol diglycidyl ether, polyethylene glycoldiglycidyl ether, or polybutadiene diglycidyl ether. Of those, ethyleneglycol diglycidyl ether is most preferred. The dried collagen product isimmersed in an ethylene glycol diglycidyl ether-containing solution,followed by deaeration under reduced pressure (1 minute to 1 hour) andsubsequent stirring (1 hour to 48 hours). After that, the product issubjected to washing with an ion exchange water and neutralization of anunreacted group with a neutralizing solution, and then dried.

In the present invention, the insoluble treatment is performed so thatthe physical strength, in particular, stress of the collagen sponge isequivalent to that of a cartilage tissue. In daily life, the cartilagetissue has a deformation ratio (strain) of about 10% when compressed ina vertical direction and has a stress under the condition of from 7 kPato 30 kPa. Therefore, conditions for the insoluble treatment areadjusted so that the stress of the substrate of the present invention isalso from 7 kPa to 30 kPa when loaded with 10% strain. The collagensponge of the present invention has biocompatibility, but the insolubletreatment also provides an effect of preventing early decomposition andabsorption of the substrate in vivo.

(Applications of Collagen Sponge)

The collagen sponge of the present invention may be used for cartilagetreatment. The collagen sponge of the present invention is suited forbeing implanted into a living body as a reinforcement agent orsubstitute for a cartilage tissue. The collagen sponge of the presentinvention may be implanted as it is into a living body, or may beimplanted after cells have been inoculated thereinto and cultured priorto the implantation. In the culture, the cells may be cultured while aload similar to that to be applied to a cartilage tissue of a livingbody is applied. The collagen sponge of the present invention hastensile strength and compressive strength (stress) equivalent to thoseof a cartilage tissue into which the collagen sponge is to be implanted,and has no unevenness in structure and strength, and hence can reduce amechanical load to be applied to a tissue around the tissue into whichthe collagen sponge is implanted, and can also maintain a pore structurefor allowing cells to infiltrate thereinto.

A method of treating a joint using the collagen sponge of the presentinvention is not particularly limited, but an example thereof isdescribed below. First, small holes (e.g., at two or three sites) aremade in a skin surface surrounding a joint in need of treatment, and atrocar is inserted into the joint. Insides of the joint and the trocarare filled with physiological saline, and an endoscope is passed throughthe inside of the trocar to gain direct access to a surgical site. Asubstitute (also called a reinforcement agent) containing the collagensponge is threaded with a suture thread and soaked in physiologicalsaline. Under the state in which the substitute is immersed inphysiological saline, the substitute is passed through the inside of thetrocar to be transported into the joint by pulling the suture thread ofthe substitute. The substitute is implanted into the damaged site of thecartilage tissue of the joint, and is sewn with the suture thread. Afterthat, the trocar and the endoscope are removed from the joint, and theholes in the skin surface are closed.

The collagen sponge of the present invention may be used for thesubstitution of cartilages throughout the body, such as articularcartilage, epiphyseal cartilage, laryngeal cartilage, nasal cartilage,costal cartilage, tracheal/bronchial cartilage, articular disc,articular meniscus, articular lip, intervertebral disc, pubic cartilage,auricle, ear canal, and epiglottic cartilage. Examples of the cartilageinclude fibrous cartilage, hyaline cartilage, and elastic cartilage. Thecollagen sponge of the present invention is preferably used for thesubstitution of fibrous cartilage formed by overlapping fibrilcomponents. Examples of the fibrous cartilage include sacroiliac joint,temporomandibular joint, sternoclavicular joint, intervertebral disc,pubic symphysis, articular meniscus, and articular disc. The articularcartilage is a tissue to be subjected to a mechanical load, and ageneral therapeutic method for treating a cartilage defect istransplantation of autologous cartilage, substitution of a fibrin clot,excision of a damaged part, transplantation of an artificial joint,medication, no treatment, or the like. The collagen sponge of thepresent invention has high biocompatibility, and hardly causes anantigen-antibody reaction when transplanted. In addition, the collagensponge of the present invention is decomposed by phagocytes or the liketo disappear in a certain period of time. When substituted in adefective portion of cartilage, in particular, a meniscus defectiveportion, the substitute allows surrounding cells to infiltratethereinto, is decomposed over time, and allows the reconstruction of thetissue along with the decomposition. The cells that have infiltratedinto the collagen sponge secrete an ex vivo matrix to regenerate thetissue. Even after the decomposition of the collagen sponge, the cellsand the secreted matrix remain, resulting in a state in which the tissuehas been regenerated.

The collagen sponge of the present invention is particularly suited foruse as a reinforcement agent for a meniscus. The meniscus is a tissuesandwiched between the articular cartilages of a femur and a tibia, issubjected to strong forces from above and below, and changes its shapein response to the forces. When merely press-fitted to a defectiveportion of the meniscus, a substitute formed of a collagen sponge fallsoff, and hence suture is required. The collagen sponge of the presentinvention can maintain its tensile strength even when exposed to a bodyfluid or water, and has such physical strength that, even when thedefective portion is sutured, the collagen sponge is not broken from aportion threaded with a suture thread to fall off, and can endure thesuture of the tissue. That is, the collagen sponge of the presentinvention can be used as a substitute in treatment involvingreconstructing the meniscus (suture or substitution). The collagensponge of the present invention is also excellent in compressivestrength, and hence the pore structure therein does not collapse andallows surrounding cells to infiltrate thereinto, and besides, thecollagen sponge does not apply a load (physical stimulus) to asurrounding tissue when transplanted into the meniscus.

A human meniscus has a crescent shape having a length of about 40 mm, awidth of about 8 mm, and a thickness of from 1 mm to 4 mm. The meniscusis a tissue whose outer edge has fibrils oriented from an anterior horntoward a posterior horn and is strong against a force applied outwardfrom the center, and whose inner edge is strong against compression.Hitherto, the reconstruction of a totally removed or totally absentmeniscus has involved using an autologous tendon for the outer edge andleaving the inner edge untreated because of the unavailability of asubstitute having strength capable of enduring compression. The collagensponge of the present invention is particularly preferably used as asubstitute for substitution for the total removal, total absence,subtotal removal, or partial absence of the inner edge. The substitutefor the inner edge needs to have a maximum size of a length of 30 mm.The collagen sponge of the present invention can provide a substitutethat, even when having a size of a length of 30 mm, has highbiocompatibility, has a uniformpore structure and uniform compressivestrength, and has tensile strength allowing suture. The collagen spongeof the present invention may be used for surgery under arthroscopicview. The collagen sponge of the present invention has strength capableof enduring suture even when impregnated with water in blood, a bodyfluid, physiological saline, or the like. In the surgery underarthroscopic view, the collagen sponge of the present invention can bepassed through the inside of a trocar by threading the collagen spongeinfiltrated with physiological saline with a suture thread and pullingthe suture thread, and can be sutured to a tissue in the state of beingimmersed in physiological saline.

The collagen sponge of the present invention is particularly suited forthe treatment of the meniscus among cartilage tissues. Therefore, it isimportant that the shape of the collagen sponge have a thickness(height) of 1 mm or more and 10 mm or less, preferably 3 mm or more and5 mm or less. It is preferred that the shape of the collagen sponge havea length of 1 mm or more and 50 mm or less, preferably 5 mm or more and30 mm or less, and a width of 1 mm or more and 50 mm or less, preferably5 mm or more and 30 mm or less. In addition, the collagen sponge of thepresent invention can be cut/processed so as to have a shape matchingthe size of a defect of each tissue, and hence does not entail anoperation such as joining of a plurality of substitutes together. Whenthe collagen sponge of the present invention is used for substitutionfor the total removal, total absence, subtotal removal, or partialabsence of the meniscus, the collagen sponge preferably has a disc shapeor a crescent shape. When the collagen sponge of the present inventionis used for regeneration treatment of the meniscus, it is consideredthat the collagen sponge allows the meniscus to be repaired to itsoriginal size, and hence the risk of developing knee osteoarthritis canbe reduced.

The collagen sponge of the present invention may also be used forregeneration treatment of an intervertebral disc.

In addition, the collagen sponge of the present invention may be notonly applied to hitherto performed cartilage treatment, but also usedfor cartilage treatment to be developed in the future, in particular,meniscus regeneration treatment or intervertebral disc regenerationtreatment.

The present invention is described by way of Examples below. However,the present invention is by no means limited to these Examples.

EXAMPLE 1

[Production of Collagen Sponge of the Present Invention]

(1) Enzyme-solubilized collagen (atelocollagen) derived from bovine skinwas added to hydrochloric acid, and further, the mixture was subjectedto stirring and deaeration using Awatori Rentaro (trademark) ARE-310(Thinky Corporation) at a revolutional centrifugal force of 420 G and arotational centrifugal force of 25 G for 5 minutes (30 seconds×10 times)to provide an acidic collagen solution (pH 3). The stirring anddeaeration was divided into 10 times so as to prevent an increase intemperature of the collagen solution. In this Example, the operation ofthe stirring and deaeration was stopped every 30 seconds, and thetemperature of the collagen solution was monitored and confirmed not tobe more than 30° C. The collagen concentration in the solution wasmeasured by the Kjeldahl method, and the collagen concentration in thesolution was adjusted to 80 mg/ml. When the concentration was less than80 mg/ml, the concentration was increased by adding enzyme-solubilizedcollagen.

7 ml of the collagen solution having a collagen concentration of 80mg/ml was dispensed into a cup made of polyacetal, and further, wassubjected to stirring and deaeration treatment using Awatori Rentaro(trademark) ARE-310 (Thinky Corporation) at a revolutional centrifugalforce of 516 G and a rotational centrifugal force of 0.14 G (deaerationmode, 2,200 rpm) for 90 seconds.

(2) After the stirring and deaeration, the cup having the collagensolution dispensed therein was left to stand still on a shelf and cooledfrom room temperature to −20° C. to freeze the collagen solution, andthen dried under reduced pressure for 73 hours while the temperature wasincreased over time from −30° C. to normal temperature.

(3) The dried collagen product after the completion of the freeze-drywas removed from the cup made of polyacetal, cut to a requiredthickness, and then subjected to insoluble treatment by placing thedried collagen product in a container made of PP containing ethyleneglycol diglycidyl ether serving as a cross-linking agent, and performingdeaeration under reduced pressure (10 minutes to 20 minutes), followedby stirring (30° C., 18 hours), so that the finished product had atensile strength of 1 N or more and 5 N or less and had a stress of 7kPa or more and 30 kPa or less when loaded with 10% strain.

Next, the collagen sponge after the insoluble treatment was transferredto a container made of PP containing ion exchange water, and stirred(30° C., 30 minutes∴10 times to 15 times). After that, a neutralizingsolution for the cross-linking agent was added, and the mixture wasstirred (30° C., 18 hours), transferred again to a container made of PPcontaining ion exchange water, and stirred (30° C., 30 minutes×10 timesto 15 times). Thus, washing was performed. Finally, the resultant wasdried to provide a collagen sponge of the present invention.

As Comparative Example, on the basis of a method described in PatentLiterature 3, a collagen dispersion obtained by dispersing collagen inpurified water was centrifuged at 760 G for 20 minutes instead ofstirring and deaeration treatment, and then subjected to freeze-dry andinsoluble treatment to produce a related-art collagen sponge.

EXAMPLE 2

[Confirmation of Pore Structure and Physical Strength of CollagenSponge]

A scanning electron microscope image (SEM image), tensile strength, andcompressive strength were confirmed for each of the collagen sponge ofthe present invention produced in Example 1 and the related-art collagensponge.

(1) A cross-section of each of the collagen sponge of the presentinvention and the related-art collagen sponge was observed with ascanning electron microscope. The cross-section of the collagen spongeof the present invention is shown in FIG. 1 (scanning electronmicroscope image, Bar: 100 μm) , and the cross-section of therelated-art collagen sponge is shown in FIG. 2. As apparent from FIG. 1,the structure of the surface of the collagen sponge of the presentinvention was uniform, and a dense pore structure was found. Meanwhile,the surface of the related-art collagen sponge was uniform, butindividual pores thereof were large as compared to those of the collagensponge of the present invention, and it was found that a portion inwhich collagen was present and a portion in which collagen was absent(pore) were distinct from each other.

Further, the diameters of the pores on the surface (scanning electronmicroscope image) of each of the collagen sponge of the presentinvention and the related-art collagen sponge were measured. One porewas selected randomly from the surface of the collagen sponge, the longdiameter of the pore was measured, and the long diameter was defined asthe diameter of the pore. The procedure was performed for 100 pores, andthe average pore diameter and pore diameter standard deviation werecalculated.

The collagen sponge of the present invention was found to have anaverage pore diameter of 11.85 μm and a pore diameter standard deviationof 5.89 μm. The related-art collagen sponge was found to have an averagepore diameter of 52.96 μm and a pore diameter standard deviation of24.22 μm. In addition, the collagen sponge of the present invention wasproduced by the same technique as in Example 1, and the diameters of thepores thereof were measured in the same manner as in this Example andfound to be 9.97 μm±4.85 and 15.01 μm±6.99. Meanwhile, the related-artcollagen sponge was produced in the same manner as the one described asComparative Example in Example 1, and the diameters of the pores thereofwere measured and found to be 42.08 μm±15.86 and 72.64 μm±35.33. Thus,it was confirmed that the pores of the collagen sponge of the presentinvention had smaller sizes than those of the related-art collagensponge.

(2) The stresses of the collagen sponge of the present invention and therelated-art collagen sponge were measured. By referring to thedeformation degree (10% strain) of the meniscus by a load to be appliedto the knee in daily life, the stress was measured when a scaffold wascompressed with 10% strain. The stress was measured for the collagensponge of the present invention immersed in physiological saline at 37°C. using a small desktop testing machine EZ-S (Shimadzu Corporation).

As a result, the collagen sponge of the present invention was found tohave a stress of about 10 kPa when loaded with 10% strain. Therelated-art collagen sponge was found to have unevenness in many partsand have a stress of about 18.7 kPa when loaded with 10% strain in acertain part.

(3) In accordance with the method described in FIG. 3, the tensilestrength of each of the collagen sponge of the present invention and therelated-art collagen sponge was measured. First, a cylindrical collagensponge test piece having a diameter of 30 mm and a thickness (height) of5 mm was prepared. Two sites each 5 mm away from the center on astraight line passing through the center of a smooth surface of the testpiece were each threaded with a suture thread (No. 2-0). The test piecein the state of being threaded with the suture threads was immersed inphysiological saline. The test piece was placed in a desiccator, avacuum pump was connected thereto, and degassing was performed for3minutes. The suture threads on both sides threading the test piece werefixed to a small desktop testing machine EZ-S (Shimadzu Corporation) andpulled at a rate of 10 mm/min, and a force (N) at break (at the time ofstart of breakage) in this case was measured. The thickness of the testpiece was measured with a Vernier caliper.

The results are shown in FIG. 4 and Table 1 below. All 51 test pieces ofthe collagen sponge of the present invention had a tensile strength of1.5 N or more, and hence it was confirmed that the tensile strength wasstrong in all directions. Meanwhile, the related-art collagen sponge hada mean value for tensile strength lower than that of the collagen spongeof the present invention, and a minimum value for tensile strength of0.79 N, and hence it was shown that tensile strength required incartilage treatment was not satisfied in some directions.

TABLE 1 Tensile Strength Example 1 Comparative Example Mean value (N)2.39 1.29 Standard deviation 0.42 0.32 value (N) Maximum value (N) 3.431.95 Minimum value (N) 1.54 0.79 Number of samples n = 51 n = 14

EXAMPLE 3

[Experiment on Implantation of Collagen Sponge into Living Body]

The collagen sponge of the present invention produced in Example 1 wasimplanted into miniature pigs, and was evaluated for efficacy and safetyin meniscus defect repair. A 3 mm×8 mm defect was produced in a regionin front of the medial meniscus ranging to the medial joint of each ofeight knees of eight 10- to 12-month-old miniature pigs, and asubstituted group of four knees each fitted with the collagen spongehaving the same size as the defective portion and a non-substitutedgroup of four knees were comparatively investigated 6 months aftersurgery. Quantitative evaluations based on scores were performed throughmacroscopic observation for tissue repair at the defective portion,cartilage damage, and the presence or absence of synovial membranehyperplasia, and through histological observation for the repairedtissue amount of the defective portion, a repaired tissue image,safranin O stainability, a meniscus surface shape, a meniscus innershape, and adhesion to a surrounding tissue.

The results were as described below. In the substituted group with thecollagen sponge of the present invention, tissue repair wassignificantly satisfactory in the macroscopic observation. Nosignificant difference in cartilage damage or synovial membranehyperplasia was found between the two groups. In the histologicalobservation, the substituted group with the collagen sponge of thepresent invention was significantly satisfactory. In view of this, itwas considered that the treatment for repairing a meniscus defect inminiature pigs with the collagen sponge of the present invention wassafe and efficacious.

REFERENCE EXAMPLE 1

A structural difference between a collagen sponge obtained withoutundergoing a fibrillogenesis step and a collagen sponge obtained througha fibrillogenesis step was confirmed.

The product of the present invention was used as the collagen spongeobtained without undergoing a fibrillogenesis step.

The collagen sponge obtained through a fibrillogenesis step was producedas described below. Enzyme-solubilized collagen (atelocollagen) derivedfrom bovine skin was added to hydrochloric acid to prepare a collagensolution (11.0 mg/mL, pH 3.0). The collagen solution was filteredthrough a membrane filter (pore size: 1 μm) (maximum air pressure: 4atom). After that, the collagen solution obtained by the filtration wasneutralized to be gelled (fibrillogenesis). Specifically, the collagensolution was poured into a 10 cm×10 cm tray to a height of 1 cm. Thetray was left to stand still in a sealed container (capacity: 1 L), andthe inside of the container was filled with an ammonia gas forneutralization. After hour, the tray containing the collagen gelobtained through fibrillogenesis was removed from the container. Theresultant collagen gel was subjected to freeze-dry and insolubletreatment to produce a collagen sponge.

The collagen sponge obtained without undergoing a fibrillogenesis step(product of the present invention) and the collagen sponge obtainedthrough a fibrillogenesis step were observed with a scanning electronmicroscope (SEM) and a transmission electron microscope (TEM) (FIG. 5).The collagen sponge obtained through a fibrillogenesis step was found tohave surface irregularities, and confirmed to have a unidirectionalfibrillar structure. Meanwhile, the collagen sponge obtained withoutundergoing a fibrillogenesis step had a smooth surface, and, was notfound to have a fibrillar structure.

INDUSTRIAL APPLICABILITY

The collagen sponge of the present invention has approximately constanttensile strength in any direction, and has excellent tensile strengtheven under moist condition. In addition, the collagen sponge of thepresent invention is also excellent in compressive strength, and henceis suitable for use as a substrate for implantation into a cartilagetissue defective portion. In surgery under arthroscopic view, thecollagen sponge of the present invention can be passed through theinside of a trocar by threading the collagen sponge infiltrated withphysiological saline with a suture thread and pulling the suture thread,and can be sutured to a tissue in the state of being immersed inphysiological saline. In addition, the collagen sponge of the presentinvention is useful because it is considered that the collagen spongeallows a meniscus to be repaired to its original size, and hence therisk of developing knee osteoarthritis can be reduced. Further, thecollagen sponge of the present invention has physical strength that hasheretofore been impossible to achieve, and hence the collagen sponge ofthe present invention is also expected to be used in the development ofnovel cartilage reconstruction treatment.

1. A collagen sponge, comprising a porous construct having a pore structure, the collagen sponge having a tensile strength of 1 N or more and 5 N or less in any direction.
 2. The collagen sponge according to claim 1, wherein the collagen sponge has been subjected to insoluble treatment with a chemical cross-linking agent.
 3. The collagen sponge according to claim 1, wherein the collagen sponge has an average pore diameter falling within a range of 1 μm or more and less than 50 μm.
 4. The collagen sponge according to claim 1, wherein the collagen sponge has a stress of 7 kPa or more and 30 kPa or less when loaded with 10% strain.
 5. The collagen sponge according to claim 1, wherein the collagen sponge comprises a freeze-dried product of an acidic collagen solution.
 6. A collagen sponge, which is obtained by applying insoluble treatment with a chemical cross-linking agent to a porous construct having a pore structure obtained by freeze-drying a collagen solution subjected to stirring and deaeration treatment, the collagen sponge having a tensile strength of 1 N or more and 5 N or less and having a stress of 7 kPa or more and 30 kPa or less when loaded with 10% strain.
 7. The collagen sponge according to claim 6, wherein the collagen solution comprises a collagen solution obtained without undergoing a fibrillogenesis step.
 8. A production method for a collagen sponge, comprising the following steps: (1) a step of subjecting a collagen solution obtained by mixing collagen and a solvent to stirring and deaeration treatment; (2) a step of subjecting the collagen solution to freeze-dry treatment; and (3) a step of subjecting a dried collagen product after the freeze-dry treatment to insoluble treatment.
 9. The production method for a collagen sponge according to claim 8, wherein the collagen solution in the step (2) comprises a collagen solution obtained without undergoing a fibrillogenesis step.
 10. The production method for a collagen sponge according to claim 8, wherein the collagen solution has an acidic pH.
 11. The production method for a collagen sponge according to claim 8, wherein the step (1) includes the following step (1-1) and step (1-2): (1-1) a step of preparing a collagen solution by adding collagen to a solvent, followed by stirring and deaeration treatment; and (1-2) a step of subjecting the collagen solution injected into a container to stirring and deaeration treatment, to thereby reduce air bubbles in the collagen solution.
 12. The production method for a collagen sponge according to claim 8, wherein the stirring and deaeration treatment is performed at a revolutional centrifugal force of 1 G or more and 600 G or less and a rotational centrifugal force of 0.1 G or more and 80 G or less.
 13. A substitute for cartilage treatment, comprising the collagen sponge of claim 1 as a substrate.
 14. The substitute for cartilage treatment according to claim 13, wherein the cartilage treatment comprises fibrous cartilage regeneration treatment.
 15. The substitute for cartilage treatment according to claim 13, wherein the cartilage treatment comprises meniscus regeneration treatment. 